1. Field of the Invention
The present invention relates to a cell electrical path breaking mechanism, and, more particularly, to a cell electrical path breaking mechanism which makes a storage cell, renewable by recharging, safe to use.
2. Description of the Related Art
For example, chargeable lithium ion secondary cells are widely used as electrical power sources in portable devices such as portable telephones or personal computers. In such secondary cells, organic solvent type electrolyte is injected into a cell case, which is hermetically sealed.
However, when such secondary cells are overcharged when they are recharged, or when an amount of electrical current larger than a specified amount is passed through them, an abnormality occurs in the cell, causing gas to be produced in the cell case. The production of gas increases the pressure and the temperature in the cell case. When the pressure and the temperature increase, the cell case expands and cracks, causing the electrolyte in the cell case to leak out of it, which adversely affects devices incorporating such secondary cells.
Even if the abnormality occurring in such secondary cells is not as serious as that described above, it is still necessary to stop using any abnormal cells immediately. This is because continued use of abnormal cells causes them to expand more and more, which may cause cell cases incorporating abnormal cells to rupture.
To prevent a cell from rupturing, a cell electrical path breaking mechanism is used. A conventional cell electrical path breaking mechanism used with a circular cell is shown in FIGS. 9 and 10. A cell cover 3 and an actuator 4 are mounted to a cell case 1. The cell cover 3 is mounted by welding or caulking through a gasket 2 in order to hermetically seal the cell case 1. The actuator 4 which can be displaced upward is provided below the cell cover 3.
Vent holes 3a are formed in the cell cover 3 in order to allow the portion of the air between the cell cover 3 and the actuator 4 to escape therefrom to the outside when the actuator 4 below the cell cover 3 is displaced upward and rupturing occurs.
The actuator 4 includes an annular safety valve portion 4a, which is formed by drawing or the like and which includes a protrusion 4b and grooves 4c. The protrusion 4b protrudes downward at the center portion of the actuator 4. The grooves 4c are formed radially in the surface around the protrusion 4b.
Below the actuator 4 is disposed an insulating plate 5, which has a hole 5a and a vent hole 5b formed therethrough. The hole 5a is formed to receive the protrusion 4b of the safety valve portion 4a.
Below a portion of the insulating plate 5 is disposed a lead fixing member 6, which has a hole 6a and a vent hole 6b formed therein. The hole 6a connects to the hole 5a formed in the insulating plate 5. The vent hole 6b connects to the vent hole 5b formed in the insulating plate 5.
The protrusion 4b of the safety valve portion 4a is binserted into the hole 5a of the insulating plate 5 and the hole 6a of the lead fixing member 6. A lead 7 formed of a thin metallic plate is mounted to the summit of the protrusion 4b.
The lead 7 includes a connection portion 7a which is joined to the summit of the protrusion 4b by welding or the like. It allows electrical conduction between the actuator 4 and the lead 7. The edge of the lead 7 not joined to the protrusion 4b is connected to a generating element 8 disposed below the lead 7, whereby an electrical path is formed between the generating element 8 and the cell cover 3.
The cell case 1 is filled with an electrolyte (not shown).
When an abnormality occurs in the cell, so that the pressure in the cell case 1 is increased, gas whose pressure has been increased flows from the vent holes 5b and 6b, as indicated by arrow A in FIG. 10. When this takes place, a force which tries to push the back surface of the safety valve portion 4a upward is exerted thereto.
When the force exerted on the safety valve portion 4a causes stress to be concentrated at the connection portion 7a of the lead 7, and this concentrated stress becomes larger that the shear stress of the connection portion 7a, the connection portion 7a breaks off or peels off from the lead 7. This electrically disconnects the lead 7 and the actuator 4 from each other, thereby breaking the electrical path of the cell.
When the electrical path is broken, the flow of electrical current in the cell is interrupted. This suppresses pressure increase in the cell case 1, making it possible to prevent the cell from rupturing.
The production of smaller portable telephones or any other types of portable device using such conventional cells has caused a stronger demand for smaller and thinner cells (so that they can be used in smaller portable devices).
However, in conventional cell electrical path breaking mechanisms such as that described above, when an abnormality occurs in the cell, flammable gas is sometimes produced in the electrolyte or the like. When the internal pressure in the cell increases to a value equal to or greater than a predetermined value, causing the connection portion 7a to break off from the lead 7, an arc may be generated. The arc may ignite the flammable gas, and cause an explosion.
It is necessary to construct such conventional cell electrical path breaking mechanisms so that an arc is not produced when connection portion 7a breaks off from the lead 7. However, constructing cell electrical path breaking mechanisms taking this into account limits the types of material that can be used to form the actuator 4 or the like, and the forms that the actuator 4 or the like can take, thereby limiting the freedom with which cell electrical path breaking mechanisms can be designed.